———————————–
“… no clear evidence that tropical forests have
modified their functioning in response to climate
change over the past twenty years. Indeed, these
results tend to suggest that the forests are now
rebuilding themselves after disturbances in the
past. Consequently, tropical forests will not be
able to limit the rapid rise in atmospheric
carbon dioxide levels for a long time to come. ”

“… forests are particularly susceptible to
episodes of drought. And some climate forecasting
models have predicted a reduction in rainfall
over tropical forests in the decades to come.”
——-

ScienceDaily (Apr. 1, 2008) – More than two
million trees belonging to nearly 5000 species,
growing in tropical forests spread over 12 sites
and three continents, have been monitored since
the 1980s. The aims of this major study were to
analyze the carbon storage capacity of tropical
forests and measure the effects of climate change
on how they function. This work was carried out
by an international team, coordinated by Jérôme
Chave(1), a CNRS researcher. Their results
suggest that the tropical forests studied did
indeed act as carbon sinks, but appeared to react
principally to intrinsic phenomena rather than
climate change. They also demonstrated the
complex functioning of forest ecosystems, their
vulnerability and the importance of efforts to
ensure their conservation.

Tropical forests account for nearly two-thirds of
terrestrial biodiversity and store more than half
of the carbon in the biosphere. Recent studies
have predicted that in a carbon dioxide-enriched
environment, physiological changes will affect
tropical plants; their functioning will be
modified, their biomass will increase and they
will sequester more carbon(2). Under these
conditions, rapidly-growing tree species should
be favored over slow-growing species, and
globally, the carbon sinks represented by
tropical forests should contribute to limiting
atmospheric emissions from fossil fuels.

The international research group led by the
Smithsonian Tropical Research Institute, and
coordinated by Chave, tested these hypotheses for
the first time using forestry inventories
initiated in the early 1980s. This project
involved 38 scientists from 15 countries. More
than two million trees (with a diameter of more
than 1 cm) belonging to nearly 5000 species were
inventoried. The scientists developed novel
statistical methods which, for a given species,
made it possible to estimate the biomass of trees
as a function of wood density and trunk diameter.
They were also able to define two groups of
species: rapid-growing and slow-growing. For each
studied plot, carbon assessments were performed
at the scale of the ecosystem and for both of
these groups of species.

Chave and his colleagues confirmed that carbon
storage capacity had increased significantly
during the last two decades. Ancient tropical
forests are thus indeed major carbon sinks. What
is the mechanism underlying this carbon
sequestration? At all sites but one, the biomass
of slow-growing species had increased, but not
that of rapid-growing species(3). There was thus
no clear evidence that tropical forests have
modified their functioning in response to climate
change over the past twenty years. Indeed, these
results tend to suggest that the forests are now
rebuilding themselves after disturbances in the
past. Consequently, tropical forests will not be
able to limit the rapid rise in atmospheric
carbon dioxide levels for a long time to come.

The source of these disturbances to tropical
forests, whose effects we are still seeing,
remains a mystery. One hypothesis put forward
sometimes is that man has been altering these
forests for longer than is generally thought.
However, climatic events or processes intrinsic
to the ecosystem (e.g. attacks by pathogenic
agents) could have exerted the same effects. A
more detailed study of these natural disturbances
would provide a clearer understanding of the
consequences of human activities on tropical
forests.

The observation of long-term variations at the
sites studied suggests notably that these forests
are particularly susceptible to episodes of
drought. And some climate forecasting models have
predicted a reduction in rainfall over tropical
forests in the decades to come. Thus even if
climate change was not shown by this study to
have affected the functioning of these forests,
it is probable that this factor will play an
increasingly important role in the futureŠ. It is
therefore essential to pursue such large-scale
studies and to ensure the conservation of
tropical forests.

Notes :

(1) Jérôme Chave is a research scientist in the
Laboratoire Evolution et Diversité Biologique
(CNRS/University of Toulouse 3/Ecole Nationale de
Formation Agronomique).

(2) Forest biomass is the quantity of living
material represented by trees in a forest, all
components taken into account (roots, branches,
leaves, etc.). Half of this biomass is made up of
carbon.

(3) The results obtained at just one site
contradicted this general trend, in the Sinharaja
Forest of Sri Lanka. This forest displayed a
decline in growing biomass and an increase in
rapidly-growing species. But more detailed
inspection of the results revealed that a single
species of canopy trees, called locally the
“Ceylon ironwood” (Mesua nagassarium), has seen a
dramatic decline in its population, perhaps due
to an attack by specific pathogens.